Electronic shelf labels, operating manuals, and other portable display devices have a battery in the internal power supply and use the power of the battery to drive electronic circuits or the like to display information. A certain driving voltage is required in order to operate the electronic circuits, etc. Therefore, when the output voltage of the battery drops below a certain level, a system that monitors the remaining capacity of the battery will stop use of the battery and urge the user to charge or replace the battery. For example, Japanese Kokai Patent Application No. 2000-338204 discloses a device that detects the current and voltage from a current sensor and a voltage sensor incorporated in a battery to measure the remaining capacity of the battery.
In a battery driven display device, in order to reduce the battery replacement frequency as much as possible, it is desired to use electronic circuits or displays with low power consumption. In particular, when the display content does not change frequently as in electronic shelf labels, it is appropriate to use electronic paper that only consumes power when the display content is electrically rewritten.
The remaining battery capacity measurement device disclosed in Japanese Kokai Patent Application No. 2000-338204 measures the remaining capacity in a battery for a hybrid car. It may not be suitable for portable battery driven display devices.
FIG. 1 shows the configuration of a conventional battery driven display device. As shown in this figure, display device 10 is comprised of battery 12, such as a primary coin battery, A/D converter 14 that converts the analog voltage signal output from battery 12 into a digital voltage signal, controller 16, non-volatile memory 18 that stores display data and the like, and display device 20, such as electronic paper. Controller 16 reads out the display data stored in non-volatile memory 18 and displays them on display device 20. Also, controller 16 monitors the output of A/D converter 14 and measures the end-point voltage of battery 12. Since A/D converter 14 is used to measure the voltage of battery 12, it is possible to accurately calculate the end-point voltage by detecting the absolute voltage. The cost, however, is high.
FIG. 2 shows the configuration of another conventional battery driven display device. Display device 10A shown in this figure is equipped with two comparators COMP1, COMP2 instead of an A/D converter. Each of said comparators COMP1, COMP2 has one of the inputs connected to the output of battery 12 and the other input connected to a reference voltage ref1, ref2 (ref1<ref2), and said inputs are compared. If the input voltage is lower than the reference voltage, output signal switches from L (low) level to H (high) level. Controller 16 calculates the time at which the end-point voltage of the battery will be reached or the time remaining from detection time T1 when the output of comparator COMP1 switches from L to H and detection time T2 when the output of comparator COMP2 switches from L to H. Controller 16 stops the use of the battery and recommends battery replacement on display device 20 when there is no remaining time, or when the end-point time has passed.
FIG. 3 is a diagram explaining the method for calculating the time remaining until reaching the end-point voltage by means of linear interpolation. The ordinate is voltage, while the abscissa is time. The detection time when comparator COMP2 switches from L to H is T1, the reference voltage of comparator COMP2 is ref2, the detection time when comparator COMP1 switches from L to H is T2, the reference voltage of comparator COMP1 is ref1, and the target end-point voltage is Ve. In this case, time Te for end-point voltage Ve can be derived from the point where a straight line passing references voltages ref2 and ref1 intersects end-point voltage Ve. The time remaining until reaching end-point voltage becomes Te−T2.
However, if reference voltages ref1, ref2 of comparators COMP1, COMP2 are not accurate, it is not possible to calculate the time Te until reaching the battery end-point voltage correctly. Reference voltages ref1, ref2 can be guaranteed in a range of about ±100 mV. In that case, however, as shown in FIG. 3, the detection range of comparator COMP2 becomes the range of ref2 and ref2′, and the detection range of comparator COMP1 becomes the range of ref1 and ref1′. If the detection range of the comparator has a width, the time Te for end-point voltage Ve becomes Te1, Te2, Te3, and remaining time Te cannot be measured correctly. If the measurement error is large, the user may be urged to replace the battery even when the remaining capacity of the battery is sufficient. Alternatively, the battery becomes discharged and operation ends before it is replaced, making it impossible to use the battery effectively and properly.